1. Field of the Invention
One embodiment of the present invention relates to a light-emitting element at least part of which includes an organic compound. The present invention also relates to a lighting device, a light-emitting device, and an electronic apparatus which are provided with the light-emitting element.
2. Description of the Related Art
Development of a light-emitting device using a light-emitting element in which a layer containing an organic compound is provided between a pair of electrodes and light emission is obtained by current flowing between the electrodes has been advanced. Such a light-emitting device can be reduced in thickness and weight in comparison with a display device which is currently called a “thin display device”. Moreover, such a light-emitting device is of a self-emission type, and therefore, has a high level of visibility and high response speed. Accordingly, such a light-emitting device has been actively developed as a next-generation display device, and has been partly put into practical use at present.
Such a light-emitting element can emit light of various colors depending on a material contained in a layer containing an organic compound, which serves as an emission center. Moreover, by stacking layers containing emission center materials which exhibit different emission colors, emitted light is combined, and more variations of emission colors can be obtained. In particular, the emphasis is put on white light which can be obtained by combining red light, green light, and blue light or combining emission colors which are in a relationship of complementary colors because white light is suitable for the use of a backlight or lighting, in addition to a display.
In recent years, demand for household electric appliances which consume less power has increased in consideration of problems of oil depletion or global warming. There are a wide variety of approaches to reduce power consumption. In the case of using a self-luminous light-emitting element, emission efficiency of the light-emitting element itself is increased, and thus, power consumption can be reduced. Therefore, a large number of studies regarding improvement of emission efficiency of light-emitting elements have been made (for example, see Patent Document 1).